1. Field of the Invention
The present invention relates to a semiconductor laser device for use in a semiconductor laser module suitable as an excitation light source for a Raman amplification system.
2. Discussion of the Background
With the proliferation of multimedia features on the Internet in the recent years, there has arisen a demand for larger data transmission capacity for optical communication systems. Conventional optical communication systems transmitted data on a single optical fiber at a single wavelength of 1310 nm or 1550 nm which have reduced light absorption properties for optical fibers. However, in order to increase the data transmission capacity of such single fiber systems, it was necessary to increase the number of optical fibers laid on a transmission route which resulted in an undesirable increase in costs.
In view of this, there has recently been developed wavelength division multiplexing (WDM) optical communications systems such as the dense wavelength division multiplexing (DWDM) system wherein a plurality of optical signals of different wavelengths can be transmitted simultaneously through a single optical fiber. These systems generally use an Erbium Doped Fiber Amplifier (EDFA) to amplify the data light signals as required for long transmission distances. WDM systems using EDFA initially operated in the 1550 nm band which is the operating band of the Erbium Doped fiber Amplifier and the band at which gain flattening can be easily achieved. While use of WDM communication systems using the EDFA has recently expanded to the small gain coefficient band of 1580 nm, there has nevertheless been an increasing interest in an optical amplifier that operates outside the EDFA band because the low loss band of an optical fiber is wider than a band that can be amplified by the EDFA; a Raman amplifier is one such optical amplifier.
In a Raman amplifier system, a strong pumping light beam is pumped into an optical transmission line carrying an optical data signal. As is known in to one of ordinary skill in the art, a Raman scattering effect causes a gain for optical signals having a frequency approximately 13 THz smaller than the frequency of the pumping beam. Where the data signal on the optical transmission line has this longer wavelength, the data signal is amplified. Thus, unlike an EDFA where a gain wavelength band is determined by the energy level of an Erbium ion, a Raman amplifier has a gain wavelength band that is determined by a wavelength of the pumping beam and, therefore, can amplify an arbitrary wavelength band by selecting a pumping light wavelength. Consequently, light signals within the entire low loss band of an optical fiber can be amplified with the WDM communication system using the Raman amplifier and the number of channels of signal light beams can be increased as compared with the communication system using the EDFA.
Although the Raman amplifier amplifies signals over a wide wavelength band, the gain of a Raman amplifier is relatively small and, therefore, it is preferable to use a high output laser device as a pumping source. However, merely increasing the output power of a single mode pumping source leads to undesirable stimulated Brillouin scattering and increased noises at high peak power values. Therefore, the Raman amplifier requires a pumping source laser beam having a plurality of oscillating longitudinal modes. As seen in FIGS. 15A and 15B, stimulated Brillouin scattering has a threshold value Pth at which the stimulated Brillouin scattering is generated. For a pumping source having a single longitudinal mode as in the oscillation wavelength spectrum of FIG. 15A, the high output requirement of a Raman amplifier, for example 300 mw, causes the peak output power of the single mode to be higher than Pth thereby generating undesirable stimulated Brillouin scattering. On the other hand, a pumping source having multiple longitudinal modes distributes the output power over a plurality of modes each having relatively a low peak value. Therefore, as seen in FIG. 15B, a multiple longitudinal mode pumping source having the required 300 mw output power can be acquired within the threshold value Pth thereby eliminating the stimulated Brillouin scattering problem and providing a larger Raman gain.
In addition, because the amplification process in a Raman amplifier is quick to occur, when a pumping light intensity is unstable, a Raman gain is also unstable. These fluctuations in the Raman gain result in fluctuations in the intensity of an amplified signal which is undesirable for data communications. Therefore, in addition to providing multiple longitudinal modes, the pumping light source of a Raman amplifier must have relatively stable intensity.
Moreover, Raman amplification in the Raman amplifier occurs only for a component of signal light having the same polarization as a pumping light. That is, in the Raman amplification, since an amplification gain has dependency on a polarization, it is necessary to minimize an influence caused by the difference between a polarization of the signal light beam and that of a pumping light beam. While a backward pumping method causes no polarization problem because the difference in polarization state between the signal light and the counter-propagating pumping light is averaged during transmission, a forward pumping method has a strong dependency on a polarization of pumping light because the difference in polarization between the two co-propagating waves is preserved during transmission Therefore, where a forward pumping method is used, the dependency of Raman gain on a polarization of pumping light must be minimized by polarization-multiplexing of pumping light beams, depolarization, and other techniques for minimizing the degree of polarization (DOP). In this regard it is known that the multiple longitudinal modes provided by the pumping light source help to provide this minimum degree of polarization
FIG. 16 is a block diagram illustrating a configuration of the conventional Raman amplifier used in a WDM communication system. In FIG. 16, semiconductor laser modules 182a through 182d, include paired Fabry-Pérot type semiconductor light-emitting elements 180a through 180d having fiber gratings 181a through 181d respectively. The laser modules 182a and 182b output laser beams having the same wavelength via polarization maintaining fiber 71 to polarization-multiplexing coupler 61a. Similarly the laser modules 182c and 182d output laser beams having the same wavelength via polarization maintaining fiber 71 to polarization-multiplexing coupler 61b. Each polarization maintaining fiber 71 constitutes a single thread optical fiber which has a fiber grating 181a-181d inscribed on the fiber. The polarization-multiplexing couplers 61a and 61b respectively output the polarization-multiplexed laser beams to a WDM coupler 62. These laser beams outputted from the polarization-multiplexing couplers 61a and 61b have different wavelengths.
The WDM coupler 62 multiplexes the laser beams outputted from the polarization-multiplexing couplers 61a and 61b, and outputs the multiplexed light beams as a pumping light beam to external isolator 60, which outputs the beam to amplifying fiber 64 via WDM coupler 65. Signal light beams to be amplified are input to amplifying fiber 64 from signal light inputting fiber 69 via polarization-dependent isolator 63. The amplified signal light beams are Raman-amplified by being multiplexed with the pumping light beams and input to a monitor light branching coupler 67 via the WDM coupler 65 and the polarization-dependent isolator 66. The monitor light branching coupler 67 outputs a portion of the amplified signal light beams to a control circuit 68, and the remaining amplified signal light beams as an output laser beam to signal light outputting fiber 70. The control circuit 68 performs feedback control of a light-emitting state, such as, an optical intensity, of each of the semiconductor light-emitting elements 180a through 180d based on the portion of the amplified signal light beams input to the control circuit 68 such that the resulting Raman amplification gain is flat over wavelength.
FIG. 17 is an illustration showing a general configuration of a conventional fiber grating semiconductor laser module 182a-182d used in the conventional Raman amplifier system of FIG. 16. As seen in FIG. 17, semiconductor laser module 201 includes a semiconductor light-emitting element (laser diode) 202 and an optical fiber 203. The semiconductor light-emitting element 202 has an active layer 221 provided with a light reflecting surface 222 at one end thereof, and a light irradiating surface 223 at the other end. Light beams generated inside the active layer 221 are reflected on the light reflecting surface 222 and output from the light irradiating surface 223.
Optical fiber 203 is disposed on the light irradiating surface 223 of the semiconductor light-emitting element 222, and is optically coupled with the light irradiating surface 223. Fiber grating 233 is formed at a position of a predetermined distance from the light irradiating surface 223 in a core 232 of the optical fiber 203, and the fiber grating 233 selectively reflects light beams of a specific wavelength. That is, the fiber grating 233 functions as an external resonator between the fiber grating 233 and the light reflecting surface 222, and selects and amplifies a laser beam of a specific wavelength which is then output as an output laser beam 241.
While the conventional fiber grating semiconductor laser module 182a-182d provides the multiple longitudinal modes necessary for use in a Raman amplifier, the fiber grating module of FIG. 17 is problematic in that it has a large value of relative intensity noise (RIN) which reflects large fluctuations in light intensity. As discussed above, this fluctuation in the pumping light intensity is undesirable for Raman amplification because it could generate a fluctuation in Raman gain which in turn causes the amplified signal to fluctuate. The large value RIN is especially undesirable for Raman amplifiers using a forward pumping method, where the signal light of weakened intensity and the pumping light of high intensity propagate in the same direction. Therefore, even though the conventional fiber grating laser module provides multiple longitudinal modes which allow a diminished degree of polarization as needed in a forward pumping method, the forward pumping method is not frequently used with the fiber grating module because of the high RIN of such module.
The mechanical structure of the fiber grating laser module also causes instability of the conventional pumping light source. Specifically, because the optical fiber 203 with fiber grating 233 is laser-welded to the package, mechanical vibration of the device or a slight shift of the optical fiber 203 with respect to the light emitting element 202 could cause a change in oscillating characteristics and, consequently, an unstable light source. This shift in the alignment of the optical fiber 203 and light emitting element 202 is generally caused by changes in ambient temperature. In this regard, such changes in ambient temperature also cause small changes in oscillation wavelength selected by the fiber grating 233, further contributing to instability of the pumping light source.
Yet another problem associated with the fiber grating laser module is the high loss caused by the need for an external isolator. In a laser module with a fiber grating, an isolator cannot be intervened between the semiconductor laser device and the optical fiber because the external cavity oscillation is governed by the reflection from the fiber grating. That is, the isolator would prevent the reflected light from the grating from returning to the semiconductor laser device. Therefore, the fiber grating laser module has a problem in that it is susceptible to reflection and easily influenced. Moreover, as seen in FIG. 16, a Raman amplifier system using the fiber grating module must use external isolator 60. As is known in the art, this isolator presents a relatively high loss to the pumping light due to a connection the collecting lens and output fiber of the external isolator.